Apolipoprotein A-I (apoA-I) is the major protein component of high-density lipoprotein (HDL) particles found in human blood. High levels of HDL, and especially the apoA-I content of HDL, strongly correlate with a reduced risk of atherosclerosis, and hence a reduced risk of coronary artery disease. ApoA-I also plays a key role in the assembly of HDL particles by activating the plasma enzyme lecithin-cholesterol acyl transferase (LCAT). We need synchrotron radiation to collect high-resolution data as this structure has profound implications for how apoA-I binds to lipid in HDL.